(1) Field of the Invention
The present disclosure relates to an ultrasound diagnostic device and a control method for the same. In particular, the present disclosure is related to receive beam forming in an ultrasound diagnostic device.
(2) Description of the Related Art
Typically, an ultrasound diagnostic device transmits ultrasound towards the inside of a subject via an ultrasound probe (referred to in the following as a “probe”), and receives reflected ultrasound (an echo) via the probe. The reflected ultrasound is generated within the subject due to tissues in the subject having different acoustic impedances. Further, an ultrasound diagnostic device generates an ultrasound tomographic image based on electric signals acquired through the reception of the reflected ultrasound, and displays the ultrasound tomographic image on a monitor (referred to in the following as a “display unit”). An ultrasound tomographic image shows the structures of tissues inside the subject. Ultrasound diagnostic devices are widely used for the shape diagnosis of subjects, for having low invasiveness and achieving real-time observation of tissues through tomographic images and the like.
A typical method applied in conventional ultrasound diagnostic devices for receive beam forming (i.e., forming signals based on received reflected ultrasound (echo signals)) is so-called delay-and-sum (DAS) beam forming. One example of delay-and-sum beam forming can be found disclosed in pages 42-45 of “Ultrasound Diagnostic Device”, written by Masayasu Itou and Tsuyoshi Mochizuki and published by Corona Publishing Co., Ltd (Aug. 26, 2002).
FIG. 24 is a schematic illustrating receive beam forming in a conventional ultrasound diagnostic device. The conventional ultrasound diagnostic device illustrated in FIG. 24 includes a probe 201 and a receive beam former 202. The probe 201 includes a plurality of ultrasound transducer elements (referred to in the following as “transducer elements”) 201a that receive ultrasound reflection (echo signals) from the subject. The receive beam former 202 electrically converts the reflected ultrasound received by the transducer elements 201a into analog electronic signals, converts the analog electronic signals into digital electronic signals through some amplification and A/D conversion, and performs delaying and summing of the digital electronic signals. The receive beam former 202 includes an adding unit 2022, and a plurality of delaying units 2021 each associated with a different one of the transducer elements 201a. Specifically, each of the delaying units 2021 performs amplification, A/D conversion, and delaying with respect to an electric signal based on reflected ultrasound obtained by the corresponding transducer element 201a. Further, the adding unit 2022 generates an acoustic line signal by summing electric signals obtained through such processing by the delaying units 2021. The receive beam former 202 outputs the acoustic line signal so generated. Typically, the delay amount that each delaying unit 2021 applies is based on the distance between the corresponding transducer element 201a and a transducer element located along the central axis of the transmitted ultrasound beam. Specifically, suppose that: P denotes a measurement point that corresponds to a given position within the subject and that is located along the central axis of the transmitted ultrasound beam; c denotes a transducer element that is closest to the measurement point P; dc denotes the distance between the measurement point P and the transducer element c; m denotes a transducer element other than the transducer element c; dm denotes the distance between the measurement point P and the transducer element m; and Cs0 denotes standard ultrasound velocity within the human body. Here, the time point at which reflected ultrasound from the measurement point P arrives at the transducer element m is later than the time point at which reflected ultrasound from the measurement point P arrives at the transducer element c by a delay amount d/Cs0, which can be calculated by dm/Cs0−dc/Cs0 (refer to FIG. 25A). Thus, by calculating the time point at which reflected ultrasound from the measurement point P arrives at the transducer element c, the time point at which reflected ultrasound from the measurement point P arrives at the transducer element m can be calculated based on the delay amount d/Cs0, which indicates the amount of delay with which reflected ultrasound from the measurement point P arrives at the transducer element m. As such, each delaying unit 2021 specifies a receive signal for the corresponding transducer element 201a by considering the delay with which reflected ultrasound arrives at the corresponding transducer element 201a, and the adding unit 2022 generates an acoustic line signal by summing the receive signals specified by the delaying units 2021 (refer to FIG. 25B).
However, ultrasound velocity in the examination-target part of the subject may differ from the standard ultrasound velocity, depending upon tissue composition. FIGS. 26A and 26B respectively illustrate different velocities Cs1 and Cs2. When ultrasound velocity in the examination target part differs from the standard ultrasound velocity in such a manner, a difference in phase may still be present between receive signals even after delaying is executed with respect to the receive signals, which brings about an image blur in the acoustic line signal acquired through summing the receive signals. In connection with this, Japanese Patent Application Publication No. 2010-119481 discloses one example of technology of setting a plurality of reference areas in an ultrasound scan plane, and determining whether or not the value of ultrasound velocity used in delay-and-sum processing is appropriate based on evaluations of the reference areas.